WPAN [Wireless Personal Area Network] networks are known for some years; a PAN [Personal Area Network] network can be defined as a computer network for communicating among devices close to one person; a WPAN network is a PAN network using wireless short-range communication technologies.
A communication technology which is very often used for implementing a WPAN network is ZigBee.
One of the main and recent applications of WPAN networks is WSN [Wireless Sensor Network] networks.
In a WPAN network the key components are the nodes of the network, also called devices. In general, a WPAN network may comprise a mixture of mains powered devices and battery powered devices; battery powered devices are designed to limit their energy consumption so to assure a long lifetime to their batteries. Providing efficient use of energy in WSN networks is particularly important in order to achieve long-term deployment of applications since the sensor network nodes may not be easily recharged or replaced when the energy of their battery is over.
The component of a node of a WPAN network which is primarily responsible for energy consumption is the radio transceiver (both when it transmits and when it receives); the typical and effective way of reducing energy consumption in an asynchronous WPAN network (a network wherein the nodes do not have a synchronized clock and therefore do not transmit and receive synchronously) is to use “duty-cycling”, i.e. to let the radio transceiver of the devices operate intermittently for short intervals of times; in this way, the operation of each node is a periodic (fixed transceiver operation period) sequence of a (short) awake interval and a (long) sleep interval. Of course, this complicates the communication protocols used in WPAN networks.
From the prior art, there are known asynchronous WSN networks wherein all (or almost all) the nodes are battery powered and therefore are designed to limit energy consumption and wherein special MAC protocols are used to limit energy consumption of the radio transceivers.
The article by J. Polastre et al, “Versatile Low Power Media Access for Wireless Sensor Networks”, SenSys 2004, November 2004, describes in detail one of such MAC protocols called “B-MAC” based on “preamble sampling”. According to this protocol, when a sender node has data to transmit, it transmits a preamble that lasts at least as long as the sleep period interval of the receiver node (this duration is referred to as “preamble length”); when the receiver node wakes up (this happens according to a period referred to as “check interval”), it detects the preamble and stays awake to receive data. This article also mentions the use of a set of bidirectional interfaces that allow an application to change the “check interval” and the “preamble length” in order to optimize energy consumption, latency and throughput and adapt to changing network conditions.
The article by M. Buettner et al, “X-MAC: A Short Preamble MAC Protocol for Duty-Cycled Wireless Sensor Networks”, SenSys 2006, November 2006, describes in detail another of such MAC protocols called “X-MAC” based on “preamble sampling” and a sequence of short fixed-length preambles. According to this protocol, when a source node has to transmit an information packet, its transmitter transmits a series of short and fixed-length preambles, each containing the address of the destination node; small pauses between preambles permit the receiver of the destination node to awake (according to its own internal operation schedule), to detect the preamble and to send an acknowledgment that stops the sequence of preambles and signals the availability of the destination node to receive data; non-destination receivers which overhear the strobed preambles can go back to sleep immediately, rather than remaining awake for receiving data. This article also describes an adaptive algorithm which can be used to dynamically adjust duty-cycle parameters in order to optimize for energy consumption per packet, latency or both.
A different approach for reducing energy consumption is disclosed by U.S. Pat. No. 7,035,240; this patent deals with a method and network architecture for implementing an energy efficient network. The network includes a plurality of nodes that collect and transmit data that are ultimately routed to a base station. The network nodes form a set of clusters with a single node acting as a cluster-head. The cluster-head advertises for nodes to join its cluster, schedules the collection of data within a cluster, and then transmits the data to the base station. A cluster can intelligently combine data from individual nodes. After a period of operation, the clusters are reformed with a different set of nodes acting as cluster-heads. The network provides an increased system lifetime by balancing the energy use of individual nodes.